Modern vehicles commonly include hardware that determines the location of a vehicle and then correlates that location with a position on a roadmap. Early GNSS navigation included a purposefully-inserted error component that left received GNSS signals somewhat inaccurate. Rather than using GNSS signals for navigation, vehicles relied on a dead-reckoning system that received information from a plurality of vehicle sensors to determine vehicle speed and direction. Given a vehicle's initial position, the vehicle could calculate a displacement vector that is added to the initial position to determine the vehicle's new location. The new location can then be identified on a two-dimensional map using map-matching techniques that correlate the position of the vehicle with a road included on a map.
As vehicle navigation systems evolved, vehicles began augmenting the dead-reckoning system with GNSS receivers that received signals from a plurality of space-based GNSS satellites, such as those implementing the global positioning system (GPS) system. Present vehicle-based GNSS receivers calculate vehicle position, speed, or direction in a two-dimensional plane as a vehicle moves along a road. That is, the GNSS receivers calculate latitude and longitude coordinates, speed, and a directional heading and can translate this data into a vehicle position on a two-dimensional map. The GNSS receivers have been used alone or with location information generated by the dead-reckoning system to determine vehicle location.
In most areas, the dead-reckoning augmented with information from the GNSS receiver provides an accurate indication of vehicle location. However, some areas have roads that overlap each other on the two-dimensional plane. For example, some roads may have a lower road and an upper road that is elevated and at least partially overlaps the lower road. A vehicle located on the upper road would have similar latitude and longitude coordinates as a vehicle located on the lower road. When current vehicle-based GNSS receivers or dead-reckoning systems determine a vehicle location in the two-dimensional plane, this location does not indicate whether the vehicle is on the upper or lower road. Road geometry and navigational directions can vary depending on whether a vehicle is on an upper road or a lower road. For example, exits may lead from the right side of the road versus the left side of the road and without accurately identifying the correct road on which the vehicle is located, the driver could receive inaccurate directions.